The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. How does the chemical composition of groundwater change over time? Why is "hard water" hard, and saline water saline? (A)

Groundwater refers to water found beneath the Earth's surface in soil pore spaces and fractures of rock formations. It is an essential water source for human consumption, agriculture, and other purposes. The chemical composition of groundwater depends on various factors such as geological setting, residence time, and water-rock interactions.

The chemical composition of groundwater changes over time due to several factors, including: 1. Evaporation and precipitation: When water evaporates or precipitates, it can dissolve various minerals and substances, altering the groundwater composition. 2. Water-rock interactions: Water dissolves the minerals and ions present in rocks while passing through them, leading to changes in groundwater composition. 3. Anthropogenic activities: Human activities, such as agriculture, industrial operations, and waste disposal practices, can contribute to changes in the groundwater composition. 4. Residence time: The duration of water remaining underground can impact its chemical composition, as the longer it stays underground, the more water-rock interactions will take place.

Hard water is water that contains high concentrations of dissolved calcium and magnesium ions, making it "hard" for daily use. It can cause scale build-up in pipes, appliances, and other water-using equipment. The main reasons for hard water are: 1. Geological formations and rock types: When water passes through rock formations rich in calcium and magnesium minerals, it can dissolve these minerals and lead to hard water. 2. Water-rock interactions: As water interacts with rocks containing calcium and magnesium minerals, it can dissolve these minerals and increase the hardness of water.

Saline water is water that contains high concentrations of dissolved salts, particularly sodium chloride. The salinity of water can make it challenging to use for drinking, agriculture, and other purposes. There are two main reasons for saline water: 1. Geological formations and rock types: Water passing through rock formations containing soluble salts, such as halite and gypsum, can dissolve these minerals, leading to saline water. 2. Evaporation: When water evaporates, salts and other solutes are left behind, increasing the salinity of the remaining water. In conclusion, the chemical composition of groundwater changes over time because of several factors, including evaporation, precipitation, water-rock interactions, anthropogenic activities, and the residence time of the water. Hard water is hard due to high concentrations of dissolved calcium and magnesium ions, while saline water is saline because of high concentrations of dissolved salts, particularly sodium chloride.